One car horn is seriously not enough

I always thought one horn was enough, i was mistaken  

I could put it to good use. So yesterday night I had this lady in her car in front of me at a red light. Not sure if she was distracted or something be show slowly rolled backwards while we were waiting there at the lights. When she was about 10inches away from my bumper I hit the horn just so she wouldn't do any damage to her car by rolling into me. Not that I care much about my steel bumper but much more about the condition of her car. 

Well, unfortunately the signal just turned green at this very moment (when I used the horn) and so she waved me her middle finger and went off. 

See, with the three horn concept there wouldn't have been any form of misunderstanding. I would just have used the little ... FU*CKIN TRAIN HORN AT HER FFS!

What's a good aftermarket horn to buy? Mine sounds a little wimpy after our last mud run.